Japanese Unexamined Patent Application Publication No. 2002-141742 (Patent Document 1) described below discloses a technique for increasing distant-field directivity of a half-wavelength dipole antenna. More specifically, Patent Document 1 discloses that a comparatively small half-wavelength dipole antenna with high front-to-back ratio (F/B ratio) can be obtained by setting the half-wavelength dipole antenna at the center in a width direction and a height direction of one or a plurality of reflective plates and setting the height of the reflective plate or plates within the range of 1.3 to 1.7 times the wavelength of a used frequency.